In recent years, efforts have been directed to modifying spherical lenses by impartation thereto of an aspheric surface for the purpose of simplifying the construction of an optical system using lenses, lightening the optical system, and enhancing the quality of images to be formed by the optical system. Since aspherical lenses entail a notably high cost for the work of processing subsequent to the step of molding, however, a need is felt for a highly accurate press molding technique which has no use for any extra grinding or abrading work. Since lenses are commodities of rich-variety small-quantity production, the need of a technique capable of molding a varying species of glass, particularly a high softening species of glass containing a rare earth element (molding temperature in the range of from 700.degree. C. to 750.degree. C.), has come to find approval.
As the characteristics which the material for a glass molding die is expected to possess,
a) ideal resistance to oxidation at high temperatures and to chemical reactions at high temperatures and excellent durability,
b) ideal resistance to glass reactions and seizure and an excellent ability of mold release,
c) sparing susceptibility to infliction such as of scratch on die surface, high strength and high rigidity at high temperatures, an ability to allow highly accurate molding,
d) resistance to fracture by thermal shock, and
e) high thermal conductivity enough to allow quick absorption of heat from glass, curtail the pressing time, and consequently enhance the productivity of the molding operation may be cited, for example.
The materials which have been used for the conventional glass molding dies include metallic materials such as stainless steel, various refractory alloy steels, and cast iron, cermets such as cemented carbide having tungsten carbide bound with nickel, and ceramics such as SiC and Si.sub.3 N.sub.4, for example.
None of the materials heretofore used in the conventional glass molding dies has fulfilled all the characteristics enumerated above. Such metallic materials as stainless steel, various refractory alloy steels, and cast iron, for example, are deficient in resistance to oxidation at temperatures exceeding 600.degree. C. Since they suffer surface corrosion at temperatures exceeding 600.degree. C., the molding with a die made of such a metallic material must be carried out as kept swept with N.sub.2 gas. The dies of these metallic materials, therefore, have a problem of deficiency in productivity and durability. To make up for the deficiency in resistance to oxidation, a die made of such a metallic material and coated with platinum has been tied only to betray the disadvantage that it cannot be used for high-temperature molding because it offers poor resistance to glass reactions and succumbs to seizure at temperatures exceeding about 550.degree. C.
The cemented carbide having tungsten carbide (WC) bound with Ni has the disadvantage that the highest working temperature is 600.degree. C. by reason of resistance to oxidation. The ceramics such as SiC and Si.sub.3 N.sub.4 enjoy ideal resistance to oxidation and nevertheless have the disadvantage that they are deficient in thermal conductivity and therefore liable to generate thermal stress and, as a result, exhibit inferior resistance to thermal shock to succumb readily to fracture. Thus, the molding with a die made of such a ceramic substance must consume much time enough to preclude occurrence of a temperature gradient in the mold. The dies of ceramics moreover have a problem of inability to improve productivity because they have a small capacity for absorbing heat from the glass after the molding and necessitate a long time for the molded glass to solidify completely.
Thus, the materials which have been used in the conventional dies are invariably short of fulfilling the characteristics which the glass molding dies are required to possess.